Halftoning is a well-known and widely utilized technique for imparting a grayscale appearance to dual tone renderings of variably shaded monochromatic images (e.g., black and white images) and to dual tone color separations of variably shaded polychromatic images. It originated as an optical analog process for imparting a grayscale appearance to dual tone reproductions of continuous tone monochromatic images, but it since has been extended to provide digital halftoning processes that can be utilized by digital document processors for imparting a grayscale appearance to dual tone representations of variably shaded, scanned-in digitized images and to dual tone representations of variably shaded, computer generated synthetic images. These digitally defined images may be monochromatic or polychromatic, so it is to be understood that digital halftoning can be applied for imparting a grayscale appearance to printed and displayed renderings of monochromatic and polychromatic images. Polychromatic images typically are halftoned by halftoning each of the color separations that are provided for rendering such images.
Polychromatic imaging (sometimes simply referred to as "color imaging") is becoming increasingly important and commonplace for digital image processing applications. It, therefore, is to be understood that the present invention can be applied for embedding digital data in printed and displayed renderings of these images, typically by identically encoding the data in each of the color separations that are provided for rendering them. In other words, the encoding process that is used for embedding digital data in halftoned renderings of polychromatic images typically involves multiple iterations of the process that is employed for embedding such data in halftoned renderings of monochromatic images. Accordingly, in the interest of simplifying this disclosure, the following discussion will concentrate on the monochromatic case.
As has been pointed out previously, the functional utility of plain paper and other types of hardcopy documents can be enhanced significantly if the human readable information that they normally convey is supplemented by adding appropriate machine readable digital data to them. Input scanners can be employed for recovering this machine readable data, so the data can be employed for various purposes during the electronic processing of such documents and their human readable contents by electronic document processing systems, such as electronic copiers, text and graphic image processing systems, facsimile systems, electronic mail systems, electronic file systems, and document and character recognition equipment. See, for example, the copending and commonly assigned United States patent application of Frank Zdybel, Jr. et al., which was filed May 30, 1990, under Ser. No. 07/530,677, on "Hardcopy Lossless Data Storage and Communications for Electronic Document Processing Systems." Also see the copending and commonly assigned United States patent application of Walter A. L. Johnson et al., which was filed May 30, 1990 under Ser. No. 07/530,753 on "Form and System Utilizing Encoded Indications for Form Field Processing."
As is known, machine readable digital data can be recorded by writing two dimensional marks on a recording medium in accordance with a pattern which encodes the data either by the presence or absence of marks at a sequence of spatial locations or by the presence or absence of mark related transitions at such locations. The bar-like codes which others have proposed for recording digital data on paper utilize that type of encoding. See U.S. Pat. No. 4,692,603 on "Optical Reader for Printed Bit-Encoded Data and Method of Reading Same," U.S. Pat. No. 4,728,783 and U.S. Pat. No. 4,754,127 on "Method and Apparatus for Transforming Digitally Encoded Data into Printed Data Strips," and U.S. Pat. No. 4,782,221 on "Printed Data Strip Including Bit-Encoded Information and Scanner Contrast." Another interesting approach is to encode machine readable digital data in the shapes of the marks or "glyphs" that are written on the recording medium. See, a copending and commonly assigned United States patent application of Dan S. Bloomberg, which was filed Jul. 31, 1990 under Ser. No. 07/560,514 on "Self Clocking Glyph Shape Codes."
Glyph shape codes have the advantage that they can be designed to have a relatively uniform appearance. For instance, a simple glyph shape code suitably is composed of small slash-like marks that are tilted to the right and left at, say, .+-.45.degree. for encoding 1's and 0's, respectively. However, in some situations the more or less uniformly gray appearance of such a code may aesthetically objectionable, so there still is a need for even less obtrusive codes for encoding digital data on hardcopy documents.